The present invention relates to the generation of laser light and more specifically to the generation of tunable laser light in the ultraviolet region of the electromagnetic spectrum.
Lasers can be categorized by the regions of the electromagnetic spectrum in which they operate. These regions include the infrared (IR), visible (Vis.), and ultraviolet (UV). Each region can be further divided into regions such as the near infrared (NIR) and vacuum ultraviolet (VUV). The greatest number of laser light sources are found in the IR and visible regions of the spectrum while fewer options are available in the UV and VUV regions.
The UV region is important because photons of these wavelengths contain enough energy to break chemical bonds and because short wavelengths can be focused more precisely than longer wavelengths. UV light sources are used in applications such as spectroscopy, optical testing, medicine, machining, and lithography. For example, ArF and KrF excimer lasers are frequently used for lithography in the semiconductor industry. The short wavelengths of these lasers"" outputs enable high resolution in the resulting image. Unfortunately, these lasers have significant disadvantages including the use of toxic gasses, poor beam quality, poor power stability, and relatively broad linewidths. In large-scale applications their optical components can also be very expensive. There is, therefore, a great need for alternative light sources and calibration standards at UV wavelengths.
Common UV sources include excimer lasers and systems that rely upon the harmonic conversion of light from sources in the visible or IR regions. Excimers include ArF, KrF, and F2 gas lasers that generate light at approximately 193, 248, and 157 nm respectively. Also available is the N2 gas laser with an output near 337 nm.
Harmonic generation provides an alternative to direct generation of ultraviolet light. In this approach light is produced in the visible or IR regions and then converted to shorter wavelengths using non-linear optics such as birefringent crystals or gases. The shorter wavelengths are exact harmonics or differences between input wavelengths. Harmonic generation requires relatively high power input sources to produce higher harmonics because harmonic generation is a relatively inefficient non-linear process. Traditional input sources for harmonic generation systems include Nd:YAG, Nd:YLF, IR diode, CO2, and dye lasers.
UV sources that employ harmonic generation are generally limited by the original light sources whose fundamental outputs are typically at wavelengths above 1 micron. For example Neodymium lasers (1.064, 1.047, and 1.0535 xcexcm) produce light at fixed wavelengths and cannot be tuned to produce harmonics at desirable 248 and 193 nm outputs. Tunable systems such as dye lasers and Alexandrite lasers also have disadvantages. Dye lasers, for example, are less powerful, more complex, and more difficult to operate than direct UV sources. Alexandrite based laser systems can provide outputs between 700 and 818 nm. However, these system are severely limited with respect to their maximum power output and repetition rates.
In the present invention the output of a transform-limited Ti:Sapphire laser is modified through the use of a solid-state harmonic generation system. Using third and forth harmonic generation, the output wavelengths are tunable to wavelengths less than 333 nm and preferably between approximately 187 and 333 nm. In one embodiment the Ti:Sapphire laser is pumped via a frequency-doubled diode pumped Nd:YLF laser. Frequency selection is optionally achieved by injection locking the Ti:Sapphire oscillator with a CW external cavity diode laser. These components provide a completely solid state and tunable light source in the important ultraviolet region of the electromagnetic spectrum.
The combination of the Ti:Sapphire system with harmonic generation results in a number of unique advantages found in particular embodiments. These include the ability to stabilize the power and wavelength of the second, third, and fourth harmonics of the Ti:Sapphire oscillator by monitoring the oscillator""s output. Feedback mechanisms provide improved short and long-term stability. The Nd:YLF laser can operate at rapid repetition rates (i.e. up to and beyond 5 kHz) and, thus, reduce the peak power of individual pulses in the harmonic generation system without reducing the average power output. This increases the maximum average power output and extends the lifetime of optics within the harmonic generator and overcomes significant disadvantages of prior art systems.
Applications of the tunable UV output include optical testing and specifically testing optics intended for use in excimer lasers at 193 and 248 nm. The output can also be used as a seed source for excimer systems in order to narrow the bandwidth of the excimer output. Other applications include UV spectroscopy, laser machining, medicine (modification of living materials), and wavelength calibration.
Fifth harmonic generation is optionally employed to achieve laser output below 187 nm and in the vacuum ultraviolet.
In pulsed laser systems with a frequency conversion stage, higher average energies are typically achieved by increasing the energy input per pulse at the conversion stage. Unfortunately, higher per pulse input energies often result in shorter operational lifetimes of the conversion stagexe2x80x94such as the life time of a fourth harmonic generating crystal.
Embodiments of the invention include a system capable of producing laser light pulses at rapid repetition rates. These rapid repetition rates are optionally used to achieve higher average energies without increasing per pulse input energies to the degree required in the prior art.
In laser and optical systems there are often tradeoffs between performance parameters. For example, the current market driven performance parameters for a 193 nm laser required average powers greater than 2 mW while maintaining extended crystal lifetimes (greater than 1 hr). Prior art use of the FHG harmonics package results in 193 nm average powers below 1 mW at 1 kHz, and crystal lifetimes of less than 1 hour. As described herein, embodiments of the invention are capable of generating average powers above 2 mW with crystal lifetimes greater than 10 hours.
One embodiment of the invention includes a system for generating light with a wavelength of less than 333 nanometers. The system includes a Ti:Sapphire oscillator configured to generate a first output light, a harmonic generator disposed to receive a portion of the first output light, and configured to generate a second output light with a wavelength of less than 333 nanometers. The second output is optionally produced at a pulse repetition rate greater then 100 Hz and the Ti:Sapphire oscillator is optionally pumped by a solid state laser.
One embodiment of the invention includes a system for generating light with a wavelength of less than 250 nanometers. This system includes a Ti:Sapphire oscillator configured to generate a first output light, a fourth harmonic generating optic, and a harmonic generator, including the fourth harmonic generating optic, for receiving a portion of the first output light, and for generating a second output light with wavelength of less than 250 nanometers and equal to one fourth of a wavelength of the first output light. This system is capable of generating the second output at a pulse repetition rate greater than 1000 Hz. The second output is optionally tunable.
An embodiment of the invention includes a method of generating light with a wavelength of less than 333 nanometers including the steps of generating a first output light using a Ti:Sapphire oscillator, and generating a second light output with a wavelength of less than 333 nanometers using a harmonic generator disposed to receive a portion of the first output light.
An embodiment of the invention includes a method for generating light with a wavelength of less than 250 nanometers comprising the steps of using a Ti:Sapphire oscillator to generate a first output light, receiving a portion of the first output light with a harmonic generator, the harmonic generator including a fourth harmonic generating optic, and generating a second output light with wavelength of less than 250 nanometers using the forth harmonic generating optic. This embodiment optionally includes tuning the second output light to a desired wavelength.